The present invention relates to energizing circuits for solenoid valves, and more particularly the invention relates to an improved energizing circuit whereby when the supply of current to a solenoid valve is frequently turned on and off to frequently change the positions of the valve spool, the contacts for turning on and off the current supply are prevented from being damaged by arc discharge, and more particularly the surge voltage generated across the solenoid coil of the solenoid valve during the off period of the current supply is suppressed and the delay time in the return movement of the valve spool upon deenergization of the solenoid coil is reduced.
With a known type of solenoid valve in which current is supplied to a solenoid coil to move a valve spool against a flow force and spring force, if a switch with contacts, e.g., relay is used to interrupt the supply of current to the solenoid coil, arc discharge will be caused between the switch contacts, with the result that the life of the contacts is reduced extremely due to wear and loss of the contacts and the contacts are held in an incompletely parted condition by an arc current during the period of the arc discharge, thus causing a residual magnetic force in the solenoid coil by the current continuously flowing in the solenoid coil and thereby retarding the return movement of the spool until the residual magnetic force becomes smaller than the spring force. As a result, the change-over time of the valve will be increased thus causing a delaying phenomena. Also, immediately after the contacts have parted, a surge voltage which is several tens times the normal voltage (differing in dependence on the parting speed of the contacts) will be induced across the solenoid coil thus causing troubles, such as, dielectric breakdown of the solenoid coil or electric noise which disturbes other devices, particularly communication devices in the neighborhood.
In the past, many different measures have been proposed to prevent the occurrence of surge voltage across the solenoid coil or the occurrence of arc discharge between the contacts upon interruption of the current supply to the solenoid coil, and all of these proposed measures are of the alternative nature in that in some measures the prevention of sparks between the contacts is accompanied with an increased delay time in the return movement of the spool upon deenergization of the solenoid coil and in another measures the suppression of surge voltage is accompanied by the occurrence of spark between the contacts and an increased delay time in the return movement of the spool. For instance, a measure is known in the art in which a diode is connected in parallel with a DC solenoid coil in opposite polarity so that the voltage induced in the coil upon opening of the contacts is discharged through the diode, that is, the surge voltage produced across the solenoid coil is suppressed by the continuity of the current flow thus extinguishing the spark produced between the contacts, and this measure is disadvantageous in that although there occurs no spark of the order that can be observed visually, there actually occurs spark of the order sufficient to cause wearing of the contacts and moreover the delay time in the return movement of the valve spool is several times that obtained without using the diode.
On the other hand, another measure is known in which the ordinary varistor whose electric resistive element is mainly composed of intergranular point contacts, such as, SiC varistor is connected in parallel with a solenoid coil so as to absorb the surge voltage at a certain voltage value, and this is also disadvantageous in that in the case of a large power coil, such as, the coil of a solenoid valve, a very large back electromotive force will be generated upon the interruption of current flow with the result that it is impossible to extinguish a spark produced between the contacts by simply absorbing only that part of a high voltage of a narrow waveform which is higher than a certain voltage value and the delay time in the return movement of the valve spool remains as long as is the case without connecting the varistor.
Still another measure is known in which a capacitor is for example connected in parallel with a solenoid coil so that the back electromotive force produced upon deenergization of the coil is cancelled by the charge stored in the capacitor, and this is also disadvantageous in that the capacitor must have a capacitance value of a certain measure to prevent the occurrence of spark between the contacts and that the capacitance value of the capacitor must be large in order to suppress the surge voltage to a certain value. For example, in the case of a DC solenoid valve having a rated voltage of 24 V, if it is desired to suppress the surge voltage across the coil to 150 V, a capacitor having a relatively large capacitance of several tens .mu.F must be used to produce the desired effect, that is, if a capacitor having for example a capacitance value of 1 .mu.F is used, a surge voltage of the order of 1,000 V will be produced across the coil upon deenergization of the coil. Since voltages of opposite polarities will be applied across the capacitor during the supply of current to the coil and immediately after the interruption of the current supply, respectively, the capacitor must be a nonpolarized capacitor with the result that it is difficult to obtain inexpensively a nonpolarized capacitor of a large capacitance and it is necessary to use a large capacitor which is difficult to use as a built-in component part around the valve. Further, since a capacitor of a large capacitor must be used, when current is supplied to a solenoid coil, a large rush current flows to the capacitor and consequently such rush current cannot be ignored in cases where a plurality of solenoid coils are energized from the same power source.
Thus, with the ordinary solenoid valve, in order to change the passages for a large pressure or large flow, the spring force of a spool return spring must be selected large to change the passages upon deenergization of the solenoid coil and the flow force also increases. As a result, the attractive force of a solenoid must be large to overcome such large opposing forces and it is necessary to use a large solenoid coil, thus inevitably increasing the ampere turns and causing upon interruption of the current supply a back electromotive force which is extremely large with those of other coil devices. Consequently, if the current supply is interrupted by a contact-type switching device such as relay, many peculiar problems will be caused, namely, the contacts will generally be made inoperative after the expiration of about one half the rated life of the switching device, not only the delay in the opening of the contacts but also the delay in the return movement of the valve spool must be taken into consideration, and so on.
On the other hand, while it is essential to guarantee a life over a long period of time for hydraulic equipment used as industrial equipment, there are many cases where such hydraulic equipment is incorporated in one unit along with an electric component, such as, electric motor, relay or timer which controls the operation of the hydraulic equipment, with the result that the electric component is considered as a part of the hydraulic equipment and it is necessary to guarantee a life for the unit in consideration of the hydraulic equipment as well as the electric component. However, with the electric components other than electric motors, such as, relays and timers, generally the life of their contacts, etc., is short as compared with that of the hydraulic equipment so that presently it is the usual practice to select a relatively short guaranteed life in consideration of the short life of the electric component or alternatively a life is guaranteed in consideration of the relatively long life of the hydraulic equipment and the replacement of a relay or the like without cost is made obligatory within the guaranteed period.